Click on tiles to cycle through numbers. If "Show Legal Only" is selected, then tiles only cycle through legal numbers only.

Click a checkbox to "lock" the number on the corresponding tile.

"Solve #1" finds tiles with only one valid number.

"Solve #2" finds numbers for which only one tile is valid.

"Clear" clears all unlocked tiles (after confirmation)

The old version is scalable, but the locks do not overlay the tiles.